<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
    
</body>
<script src="js/itcast.core.js"></script>
<script src="js/itcast.dom.js"></script>
<script src="js/itcast.ctor.js"></script>
<script>
    var arr = [],
        push = arr.push;
    Itcast.fn.appendTo = function(dom){
        dom.appendChild( this[ 0 ] );
        return this;
    }
    
</script>

<script>

    I('<span>span</span>').append( document.body );



</script>


</html>